Star tracker detector having a partial memory section

ABSTRACT

A tracker for stellar objects including stars is provided. The star tracker includes a charge coupled device having an image section and a memory section. The image section is useful in obtaining acquisition information related to determining the presence of one or more stars. The image section is also useful in obtaining location information useful in determining the position of each such star. The image section also obtains discardable information, such as information related to dark current and background star generated charge. The memory section receives the location or acquisition information from the image section. Such information is stored in significantly fewer lines and cells in the memory section than the number of lines from which such information was obtained in the image section. When storing location information in the memory section, at least one guard line can be provided intermediate the location information and the discardable information.

FIELD OF THE INVENTION

The present invention relates to obtaining information using a chargecoupled device (CCD) and, in particular, obtaining location oracquisition information using one or more stars in which the CCD has amemory section that has fewer lines and cells than an imaging sectionthereof.

BACKGROUND OF THE INVENTION

In many aerospace related systems having orbiting space basedsatellites, the satellites must be in particular orientations to performtheir intended tasks. For example, telecommunication satellites mayrequire that their antennas are positioned for appropriate transmittingand receiving of wireless communications. Additionally, spaceexploration satellites may require very precise satellite orientationfor high resolution imaging of celestial bodies. All inertial guidancesystems drift with time and their calibration needs to be periodicallyupdated. Reckoning from stars with a tracker can provide thiscalibration with the required precision.

A satellite's position and/or orientation may be determined by varioustypes of inertial guidance systems such as a laser gyroscope. However,such inertial guidance systems are only useful for positioning andorienting an orbiting satellite up to a certain degree of accuracy. Whenfurther accuracy is desired, other techniques may be required. Inparticular, it would be desirable to enhance satellite position accuracyby using a real time star tracking system that tracks the images ofpredetermined stars. That is, such a star tracking system may beutilized to fine tune the position and/or orientation (collectively,denoted “positioning information”) of an orbiting satellite, wherein aseparate inertial guidance system supplies initial estimates of suchpositioning information. Moreover, when installed on a satellite, itwould also be desirable for such a star tracking system to be of reducedsize, weight and power consumption. Moreover, it would be also desirablethat such a star tracking system be capable of tracking a plurality ofstars simultaneously.

SUMMARY OF THE INVENTION

The present invention is a star tracker that includes a novel frametransfer CCD, wherein the memory section is substantially reduced insize, and further wherein there is only a minor increase in the parallelcharge transfer inefficiency (CTI) over that of a full frame CCD.Moreover, the present invention allows high update rates (frame rates)with low actual pixel read rates and hence reduced bandwidth demands.Additionally, the present invention eliminates the perturbing influenceof varying image smear on determining stellar image centroids byproviding a substantially uniform smear under all star trackingscenarios. Also, the present invention reduces the image smeargenerating interval. Thus, the present invention provides high framerates with low to moderate pixel read rates, and hence low bandwidthswhich translate to low noise levels, thereby making possible highsignal-to-noise ratios. Further, the present invention generates only asmall thermal load increase over that of a full frame CCD.

The tracking device of the present invention includes a novel chargeframe transfer coupled device (CCD) having an imaging area forintegrating (i.e., capturing) images thereon, and a reduced size memorysection into which each such image is transferred and compacted therein.Subsequently, each image in the reduced memory section may be read outof the CCD and processed while a next image is being integrated in theimaging area. In particular, the novel CCD outputs the compacted imagedata from the reduced memory section to tracking modules for furtherprocessing so that stars may be accurately tracked by accuratelygenerating centroids of their images.

The CCD included in the present invention is a frame transfer typehaving a split memory section. The split memory section has a firstsequence of rows of charge collecting cells (CCD cells) for storing afirst portion (e.g., an upper half) of an image obtained from a firstpart of the CCD imaging area. Note that this first sequence of rows areat a first (e.g., top) end of the CCD imaging area. Additionally, thememory section has a second sequence of rows of charge collecting cellsat an opposite end of the CCD imaging area for storing a second portion(e.g. a lower half) of the image obtained from a second part (e.g., alower half) of the CCD imaging area. In one embodiment, each of thememory section first and second portions contain half of the cellcharges (i.e., pixels) collected in the CCD imaging area. Further, therows of the CCD imaging area cells for the top half of the imaging areamay be parallelly shifted into the first portion of the memory section,and the bottom half of the rows of the CCD imaging area may beparallelly shifted into the second portion of the memory section.Accordingly, it is an aspect of the present invention that byconcurrently shifting each half of the imaging area into itscorresponding portion of the memory section, the latency time for imagetransfers to the memory section is reduced from that of a CCD having amemory section at only a single end of the imaging area.

Although the total number of memory section charge cell rows for thepresent invention is substantially fewer in number than the number ofsuch rows in the imaging area, the present invention is able to providerapid transfer of an entire image from the imaging area to the reducedsize memory section in a manner such that all the desired star trackinginformation within each image is preserved in the reduced size memorysection. That is, all such desired star tracking information can berepresented within a reduced size memory section since typically: (a)only a small number of stars are simultaneously tracked (e.g., less thanor equal to 4), and (b) each star's image lies within, for example, arelatively small square (also denoted “track-box”) of charge collectingcells of the imaging area (i.e., such a track-box is “small” compared tothe total size of the imaging area). Thus, by coalescing (or “binning”as it is referred to in the art) rows of image charges that do notintersect such track-boxes, a substantially reduced memory sectionbecomes adequate for the star tracking task. That is, the presentinvention shifts images from the imaging area into the memory sectionwhile concurrently: (a) binning the rows of pixels (denoted also “pixellines”) not intersecting any track-box, and (b) not binning image areapixel lines that intersect at least one track-box. Moreover, thereduction in the memory section has the added benefit that the chargetransfer inefficiency (CTI) of the CCD is reduced in that each charge(i.e., pixel) is transferred a reduced number of times in comparison toa memory section that is substantially the same size as the imagingarea.

Additionally, it is an aspect of the present invention to includemodules for generating one or more “guard” lines of pixels between eachbinned pixel line and each non-binned pixel line in the memory section,wherein such guard lines are intended to have relatively little chargetherein and therefore function as insulators for inhibiting the chargeleakage between binned and non-binned lines in the memory section byproviding a place to accumulate dark current and any other backgroundsignal charge.

It is a further aspect of the present invention that during the trackingof stars, there are one or more modules provided for both determining amost recent centroid of each star image being tracked, and predicting asubsequent most likely centroid for each such star image. In fact, suchpredicted centroids are used to determine (the centers of) subsequenttrack-box positions in a next image integration. Further, note that suchtrack-box prediction modules may require position, orientation andangular rotation rate information from the satellite in which thepresent invention is incorporated to correctly predict where new starimage centroids are likely to be.

When a star tracker according to the present invention is operatingabove the earth's atmosphere, almost all of the field of view (FOV) issubstantially dark with the exception of a few stellar images that aresufficiently bright so that they can be tracked. It is common for CCDsto generate what is known as “dark current” which is noise that candetract from accurately identifying and/or determining the position ofstellar images. For example, if the average dark current level is 100electrons per CCD cell, the added noise is the square root of 100, i.e.,10 electrons. This noise decreases the overall signal-to-noise ratio forstars being tracked. The average dark current is accumulated at aconstant rate above the earth's atmosphere. Thus, this dark current issubstantially a function of the CCD exposure or integration time, andreadout times, as one skilled in the art will understand. Further, thedark current rate is strongly dependent on the CCD temperature. Thus,cooling a CCD serves to reduce the influence of the dark current.Additionally, within the CCD structure, the discontinuity of the crystallattice at the silicon-silicon dioxide interface is also a majorcontributor to the overall dark current, as one skilled in the art willunderstand. It is, thus, common practice to “tie-up” dangling siliconlattice atom bonds with a hydrogen annealing process as one skilled inthe art will understand. Moreover, it is well known that exposure tohigh energy particle radiation, particularly protons, such asencountered in space, increases the dark current generation rate. It isbelieved that such radiation partially reverses the effect of hydrogenannealing. Accordingly, the dark current level at the end of a CCD'suseful life must be accommodated for in the design of a star tracker.

When transferring an image from the imaging area to the memory sectionin a CCD according to the present invention, there will be many moreparallel charge shifts than there are corresponding shifts in thereduced memory section. Thus, most of the generated dark current andbackground stellar generated charge initially occurs in the imagingarea, and is driven toward one of the interfaces with the memory sectionduring transfer thereto. Since one embodiment of the CCD provided by thepresent invention utilizes a three phase clocking structure (i.e., thereare three parallel clocking electrodes per row of charge cells), if eachmemory section cell row that is an interface to the imaging area has itsfirst memory phase electrode always “parked” at the high clock potentiallevel, each such interface cell row acts as an accumulator that cancollect a plurality of consecutive rows of image charges (also denoted“pixel lines”) of such dark current and stellar background noise. Suchsumming of charges is also denoted herein as “binning.” Thus, asmentioned hereinabove, a few “extra” cell rows in the memory section inaddition to those needed to accommodate the track-box(es) are used tocollect the dark current and undesired background signal. Further, thedark current and undesirable background signals can be divided betweenmore than one consecutive pixel line if so needed. Thus, the number ofcell rows in the memory section is dependent upon: (a) the dimensions ofthe track-boxes, (b) the number of stars to be simultaneously tracked(i.e., the number of track-boxes), (c) the total number of pixel linesused for binning, and (d) the total number of guard band lines needed toinsulate the track-box images in the memory section from one anotherand/or from pixel lines used for binning (i.e., having accumulated darkcurrent and background noise).

Various design considerations and/or operating constraints may beprovided for the manufacturing of the present invention. In particular,the exposure time for the imaging area to capture an image must be setso that there is sufficient time to accumulate enough signal charge toachieve an adequate signal-to-noise ratio on the dimmest star to betracked. Additionally, the amount of exposure time is also dependentupon the high dynamics of satellite roll, pitch and yaw, since suchmovements result in image smear which can result in unacceptablecentroiding error, if the exposure time is too long. A reasonable ruleof thumb in determining exposure time for the present invention is thatno star image in the imaging area be allowed to move more than its widthin any given exposure time. However, note that the width of such starimages may be artificially increased in the present invention byutilization of a defocusing lens which slightly defocuses stellar lightonto the imaging area. Note that such defocusing has been found tofacilitate angular interpolation so that sub-pixel accuracy of starimage centroids can be obtained.

Another design decision to be determined when manufacturing the presentinvention has to do with purging the memory section of pixel lines thatdo not contain useful data. In particular, even though the images arecompressed into a reduced number of pixel lines in the memory section,many of these lines do not contain useful data. However, each such pixelline of the memory section must be either read out or somehow purged.Purging can be accomplished in one of two ways. In a first way, thepixels not useful for determining star image centroids are transferredto one of the pixel read out serial registers, also included in thepresent invention, and subsequently the unuseful pixels are clocked to acharge detection node for purging. In a less standard second way, suchpurging is performed via a “dump drain” connected to each such serialregister. Thus, by “opening” a dump gate and transferring theunnecessary pixels from the memory section to a serial register, thepixels are automatically dumped into the dump drain. However, thissecond way comes at a price since one preferred embodiment of thepresent invention includes serial registers that are three-phased (asone skilled in the art will understand), three layers of polysilicon areused in the manufacturing process. However, in providing a dump gate, afourth polysilicon layer is required, and such a layer is not standardin the electronics industry. Additionally note that a fast purge ofunused pixels can be accomplished by holding a reset transistor at thedetection node in an “on” state, and fast shifting the connected serialregister if there is no dump drain, as one skilled in the art willunderstand.

In addition to tracking objects such as stars, the present inventionalso includes novel techniques for acquiring or identifying new starimages to be tracked. In particular, the present invention anticipatesor predicts which stars designated for tracking are likely to be imagedon the CCD imaging area, and subsequently switches to a “staracquisition” mode wherein only imaging area cells in swaths alongcertain edges of the imaging area are processed for determining whethera candidate star for tracking is detected. Accordingly, one embodimentof the present invention includes a star database for storing thelocations of stars to be tracked along with a radiation “signature” ofeach such star. Thus, once a star tracking controller for the presentinvention receives (from, e.g., an on-board satellite inertial guidancesystem) position, orientation and angular rotation rate information, thestar tracking controller is able to interrogate the star database forany candidate stars for tracking that are likely to be detected along anedge of the CCD imaging area. Accordingly, the processing performedduring the star acquisition mode can be performed efficiently sinceswaths of cells along only two edges of the (rectangular) imaging areaneed be processed and the remainder of the imaging area ignored.

Other features and benefits of the present invention will become evidentfrom the accompanying drawings and detailed description herein below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B jointly provide a diagram illustrating the components ofthe star tracking system of the present invention.

FIGS. 2A and 2B provide a high level flowchart of the steps performed bythe star tracking system controller for tracking stars and acquiring newstars to be tracked.

FIG. 3 is a high level flowchart of the steps performed during theacquisition of new star images to be tracked.

FIG. 4 is a flowchart of the steps performed during the transferring ofan image from the imaging area to the memory section of the CCD.

FIG. 5 is a high level flowchart of the steps performed when outputting,from one of the portions of the split memory section to itscorresponding serial register, pixel lines from along an edge of theimaging area during the acquisition mode for acquiring new star imagesto be tracked.

FIGS. 6A and 6B illustrate the steps of a high level flowchart foroutputting, from one of the portions of the split memory section to itscorresponding serial register, pixels obtained from a swath of columnsof imaging area cells along an edge of the imaging area during the staracquisition.

FIG. 7 provides a high level flowchart for tracking a star imageaccording to the present invention.

FIG. 8 is a flowchart for transferring an image from the imaging area tothe memory section according when tracking star images.

FIG. 9 is a flowchart for outputting a compacted image in the memorysection to other components of the present invention such as acorresponding serial register, an A/D converter, and centroid andprediction modules.

FIGS. 10A through 10C provide an exemplary flowchart illustrating thesteps performed by the present invention during the tracking of twoparticular star images.

FIGS. 11A through 11C provide an exemplary flowchart illustrating thesteps performed by the present invention during star image acquisitionmode, wherein a particular orientation of the present invention isassumed relative to star images that move into the field of view.

DETAILED DESCRIPTION

In the block diagram of FIGS. 1A and 1B, the high level components ofthe novel star tracker 20 of the present invention are shown. The startracker 20 includes a charged coupled device (CCD) 24, wherein the CCD24 includes the following subcomponents (a) through (c):

(a) A CCD imaging area 28 for collecting electrical chargescorresponding to images to which the imaging area 28 is exposed. As oneskilled in the art will understand, the imaging area 28 includes anarray of photo-active charge collecting cells 36 (herein also denotedCCD cells), wherein each CCD cell 36 retains an electrical charge (eachsuch charge also denoted a “pixel charge” or simply “pixel”) that isindicative of the spectral radiation contacting the CCD cell. In oneembodiment, each such cell 36 is approximately 15 microns by 15 microns,and the imaging area 28 is a cell array having 512 cell rows 32, witheach such row 32 having 512 CCD cells.

(b) A memory section having two separate portions thereof, namely,memory section 42 a and memory section 42 b. These memory sections 42 aand 42 b each have a plurality of cell rows 32 used as temporary storagefor images that have been first captured or integrated on the imagingarea 28 and then subsequently transferred into the memory sections 42 aand 42 b, as discussed hereinbelow. The total number of cell rows 32 ofthe memory sections 42 a, 42 b is significantly less than the totalnumber of cell rows or lines 32 of the imaging area 28. The total numberof cell rows 32 of the memory sections 42 a, 42 b is preferably lessthan 50% of the total number of cell rows 32 of the imaging area 28 andis typically less than 75%-90% and can be less than 90%.

(c) A pair of serial registers 46 a and 46 b, wherein each of theseserial registers receives pixel charges from an adjacent line of thememory sections 42 a and 42 b. Thus, serial register 46 a receives suchpixel charge data from memory section 42 a, and serial register 46 breceives such pixel charge data from memory section 42 b. The extendedcells 54 a, 54 b of serial registers 46 a, 46 b are used to gainphysical room for needed electrical connection to such functions as theon-chip charge detection circuits 72 a, 72 b as well as memory sections32 a, 32 b clock electrodes.

Regarding imaging area 28, this area has been electronically configuredso that a first or top imaging subarea 50 a is such that pixel chargesin the cell rows 32 of this imaging subarea can be electrically shiftedsynchronously toward the memory section 42 a when an image is to betransferred out of the imaging subarea 50 a. Accordingly, the pixelcharges in the CCD cells 36 of cell row 32 a ₁, of the imaging subarea50 a are transferred into corresponding CCD cells 36 of cell row 32 a ₂of the memory section 42 a when there is a shift of cell rows 32synchronously toward the memory section 42 a. In particular, by denotingthe pixel charges within any single cell row 32 as a “pixel line,” thepixel lines within imaging subarea 50 a are capable of beingsynchronously shifted toward the memory section 42 a. Correspondingly,the imaging area 28 is also configured so that a second or bottomimaging subarea 50 b of cell rows 32 synchronously'shifts pixel linestherein toward memory section 42 b, wherein with each synchronous shiftof pixel lines, the charges in cell row 32 b ₁, of the imaging subarea50 b are transferred into the cell row 32 b ₂ of the memory section 42b.

Note that in one embodiment of the imaging area 28, each of the top andbottom imaging areas 50 a and 50 b have 256 cell rows 32 with 512 CCDcells 36 per row. Accordingly, assuming the top and bottom imaging areas50 a and 50 b are electronically activated to concurrently transfertheir pixel lines into their corresponding memory sections 42, theimaging area 28 can be completely transferred into the memory sections42 in the time required for 256 pixel line shifts.

Regarding the memory sections 42 a and 42 b, each of these memorysections has a fewer number of cell rows 32 than the number of cell rowsin the corresponding imaging subarea whose pixel charges are transferredtherein. In particular, assuming that each imaging subarea 50 a and 50 bhas 256 cell rows 32, one embodiment of the present invention has 32cell rows 32 for each of the memory sections 42 a and 42 b. Inparticular, with 32 cell rows, each memory section 42 is able to storetwo (non-compacted) 12 by 12 track-boxes along with one or moreinsulating pixel lines (denoted “guard band lines”), for insulating eachsuch track-box from other memory section pixel lines, as will bediscussed further hereinbelow. Thus, in comparison to a full frame CCD,the CCD provided by the present invention requires only a slightincrease in the parallel CTI as a result of the 32 more parallel cellrow shifts required for reading (via the serial registers 46).Accordingly, a maximum of 256+32 (=288) parallel shifts are required forreading the entire imaging area 28. However, note that it is within thescope of the present invention for each of the memory sections 42 tohave either a greater or lesser number of cell rows 32. In particular,the number of cell rows 32 in each memory section 42 is dependent uponthe number of star images to be tracked simultaneously as will bediscussed hereinbelow.

Regarding the serial registers 46 a and 46 b, the corresponding adjacentmemory section cell rows 32 a ₃ and 32 b ₃, have their pixel chargestransferred into corresponding CCD cells 36 in the adjacent serialregister 46 a and 46 bAdditionally, note that each of the serialregisters 46 a and 46 b has an extended portion 54 a and 54 b,respectively, wherein when the pixel charges within the serial registerare shifted (in the direction of arrows 58) into the correspondingextended portion 54, these pixel charges (or, their corresponding signalamplitudes) may be output to additional components of the star tracker20.

Note that in one embodiment of the CCD 24, the imaging area 28, thememory sections 42 and the serial registers 46 are adjacent to oneanother on a single silicon chip having rows of electrodes (not shown)parallel to the cell rows 32 and extending therethrough for-bothinsulating charges in individual CCD cells 36 from one another and forparallelly transferring pixel lines as one skilled in the art of CCDtechnology will understand. In particular, since the present embodimentof the CCD 24 is a three phase charge coupled device, each of theimaging subareas 50 a, 50 b, and the memory sections 42 a and 42 b havethree control lines (each set of three control lines being identified byone of the bold lines 62 in FIG. 1) for providing electrical potentialsthereon according to output by a time base generator 66 via the clockdrivers 68. That is, the time base generator 66 outputs control signalsto the clock drivers 68 and these drivers output corresponding signalson the control line sets 62 for controlling the shifting of pixel linesin the imaging area 28 and the memory sections 42 a and 42 b. Thus, thetime base generator 66 orchestrates operation of the CCD 24. Inparticular, the time base generator 66 controls all parallel and serialshifting, as well as purging, dumping and signal reading. To accomplishthis, command and control information is provided to the time basegenerator 66 from a star tracking controller 70 (described hereinbelow)which is either internal or external to the star tracker 20. Forexample, the star tracking controller 70 issues track-box locationinformation to the time base generator 66. Additionally, note that clockdriver circuits 68 provide level translation and the needed currentsource and sink capability to drive the CCD 24 clocking electrodes (notshown). Further, to achieve low read-out noise, on-chip charge detectioncircuits 72 a and 72 b are utilized to receive output charges from theextended portions 54 of the serial registers 46. Such on-chip chargedetection circuits 72 typically include a floating diffusion (notshown), and each such circuit is also referred to herein as a detectionnode and reset transistor combination, as one skilled in the art willunderstand. Note that such a floating diffusion provides a smallelectrical capacitance on the order of 0.5 e-14 Farads, which is used toconvert signal charge to voltage according to the following formula:V=q/c, A as one skilled in the art will understand. Thus, 0.5 e-14Farads translates to 3.2 micro-volts per electron. Note that the resettransistor is used to reset a capacitor within the detection node to aknown potential after each pixel read, thereby readying the detectionnode for the next charge received from its corresponding serial register46. Additionally, note that a single or multiple stage source follower(also not shown) with a gate from control transistor connected to thefloating diffusion serves to drive the off-chip load. Further, anoff-chip pre-amplifier 76 provides voltage gain to. the output receivedfrom one of the on-chip charge detection circuits. Typically, such apre-amplifier 76 is followed by an analog signal processing circuit 80such as a correlated double sampler (CDS) circuit. Accordingly, such ananalog processor circuit is used to mitigate the uncertainty of thereset transistor, thus lowering read out noise, as one skilled in theart will understand. Finally, the analog signal voltage output by eachcircuit 80 is subsequently converted to a digital signal by acorresponding A/D converter 90, and the output from each such converterprovides an adequate number of bits to handle the dynamic range ofexpected values, and provide a sufficiently low quantization noiselevel.

As mentioned hereinabove, the star tracker 20 also includes a startracking system controller 70 for controlling the operation of the startracker 20. In particular, the star tracking controller 70 outputs thecommand and control signals, via control line 74, to the time basegenerator 66 regarding how the pixel lines in cell rows 32 are to beshifted and/or binned. In particular, the star tracking systemcontroller 70 issues commands for the time base generator 66 to switchbetween a first mode of acquiring new star images to be tracked, and asecond mode of continuing to track star images that have been previouslyacquired. Additionally, the star tracking controller 70 receives, froman inertial guidance system 78, orientation information (includingangular rotation rates) of a satellite containing the star tracker 20.Note that the inertial guidance system 78 can have various embodiments(as one skilled in the art will understand); e.g., it may be a laserbased gyroscope. However, such inertial guidance systems 78 have alimited precision that is insufficient for highly precise alignment ofthe satellite wherein, for example, the satellite maintains a particularorientation toward the earth or another celestial body. Thus, thepresent invention detects smaller satellite movements, and canaccordingly be used for better satellite alignment.

The star tracking system controller 70 also controls a component(denoted the centroid and prediction modules 88) having a plurality ofcomputational modules for determining the centroid of each star imagebeing tracked, and also for determining a predicted location where eachstar image is likely to be located in a future image integrated on theimaging area 28.

Additionally, the star tracking system controller 70 receives starlocation information from a star database 94. In particular, when thestar tracking system controller 70 receives satellite orientationinformation from the inertial guidance system 78, the star trackingsystem controller is able to interrogate the star database 94 forretrieving the identities of stars that are likely to come into view onthe imaging area 28 given the satellite's orbital motion and angularrotation rates, such as row, pitch and yaw. Thus, the star trackingcontroller 70 can use such star location information for determiningwhen to direct the time base generator 66 to switch between the startracking mode and the star image acquisition mode.

There are two basic modes for the operation of a star tracker 20according to the present invention, i.e., a tracking mode and anacquisition mode. The acquisition mode is used to either find and/orconfirm where, at any particular time interval, a track box(es) in theFOV should be located, whereas the tracking mode is used to determine aprecise new location of a previously located star image (and/or thetrack-box containing the star image). To enhance overall understandingof the present invention, flowcharts for the steps performed withspecific track-boxes will first be given, then more general descriptionswill follow. Further, since description of the tracking mode may beeasier to follow, the flowchart of FIG. 10 for a specific example of thestar tracking processing will be described first. Subsequently, adescription of the flowchart of FIG. 11 for a specific exampleillustrating the acquisition mode will be provided. Following these twoexample flowcharts, more general flowcharts (FIGS. 2 through 9)describing the control processing, the tracking processing and theacquisition processing are provided.

For illustrating the tracking mode, consider the following scenario.Assume the CCD 24 is a split frame device having an imaging area 28 thatis 512 by 512, and two memory sections 42 a, 42 b that are each 32 by512 and positioned relative to the imaging area 28 as shown in FIG. 1.Also assume that there are two star images detectable in the bottomimaging area 50 b, one centered at the cell of row 100, column 80, andanother centered at the cell of row 200, column 250. Further, assumethat there is a track-box 200, respectively, about each of these starimage centers, wherein each track-box is an array of 12 by 12 cells 36(approximately as shown in FIG. 1). Given this scenario, the flowchartof FIG. 10 describes the tracking processing performed regarding theportion of an image captured in the imaging subarea 50 b. However, it isimportant to note that each of the steps of FIG. 10 equally well applyto the imaging subarea 50 a. Further, note that in one preferredembodiment of the present invention, the steps performed for imagesintegrated in 50 b are performed in parallel with those imagesintegrated on imaging subarea 50 a. Thus, for each step performed in theflowchart of FIG. 10, there is a corresponding step applicable to theimaging subarea 50 a, memory section 42 a, serial register 46 a andon-chip charge detection circuit 72 a. Accordingly, FIG. 10 commenceswith a step 1004, wherein star image signals are integrated onto theimaging subarea 50 b an effective time for providing an adequatesignal-to-noise ratio on the dimmest star image being tracked and/or sothat an amount of image smear during transfer to the memory section 42 bdoes not substantially affect the signal-to-noise ratio.

Subsequently, in step 1008, the time base generator 66 provides controlsignals, via the clock drivers 68, to the imaging subarea 50 b forparallelly shifting the first (lowest) 95 pixel lines into the first row32 b ₂ of the memory section 42 b. Note that the resulting pixel line(as well as other pixel lines similarly created) is referred tohereinbelow as an “accumulator line”. Subsequently, in step 1016, thememory section 46 b is shifted to thereby provide guard band lines asdescribed previously. Then, in step 1020, pixel lines originally in rows96 through 106 are shifted into the memory section 46 b so that thefirst track-box 200 is provided in the memory section 46 b full size.Following this, in step 1024, the memory section 46 b is shiftedindependently of the imaging area 46 b, thereby providing additionalguard band lines. In step 1028, pixel lines originally in rows 107through 194 of the imaging subarea 50 b are transferred into the firstrow 32 b ₂ of the memory section. Following this, in step 1032, thememory section 46 b is shifted to provide additional guard band lines.In step 1036, the pixel lines originally in rows 195 through 206 of theimaging subarea of 50 b are parallelly shifted into the memory section46 b, wherein the second track-box 200 is provided in the memory section46 b full size. Following this, in step 1040, additional guard bandlines are provided, and subsequently, in step 1044, the remaining pixellines originally obtained from rows 207 through 256 of the imagingsubarea 50 b are transferred into the first row 32 b ₂. In step 1046,the pixel lines of the memory section 42 b between the first track-box200 (within this memory section) and the serial register 46 b areclocked into this register and subsequently flushed or dumped withoutfurther processing. Subsequently, in steps 1050 through 1058, the pixellines having portions of the first track-box 200 are iteratively readinto the serial register 46 b, and the 12 pixels of each pixel line thatare also contained in this first track-box 200 are read (from the serialregister and through the extended portion 54 b) by the on-chip chargedetection circuit 72 b while the remainder of the pixels of each ofthese pixel lines are discarded by flushing or dumping. Subsequently, instep 1060, the next consecutive set of pixel lines not having pixels ina track-box 200 (i.e., guard band lines and an accumulation line) arenow flushed or dumped. In step 1064, the steps 1050 through 1060 arerepeated for reading the pixels of the second track box 200 and purgingall other pixels in the memory section 42 b. Thus, the memory section 42b is now clear of pixel charges, and can accept another frame of pixelsfrom the imaging subarea 50 b. Subsequently, in step 1072, the flow ofcontrol is transferred back to step 1004 for repeating these steps withanother image.

A description of a specific example of the processing performed duringthe star imaging acquisition mode will now be provided. Note that themotion of a satellite or spacecraft containing an embodiment of thepresent invention is continually bringing candidate star images into thefield of view (FOV) of the imaging area 28. In particular, motion of thesatellite or spacecraft results from orbital dynamics and can becomplicated by roll, pitch and yaw. In fact, the roll, pitch and/or yawof the satellite or spacecraft may be purposely induced to keep one ormore on-board instruments pointed at a particular object, or to acquiresightings of a particular celestial object. Since such motions can be atleast roughly measured by the inertial guidance system 78 (FIG. 1), starimages that are candidates for being tracked will enter the field ofview of the imaging area 28 along one of its leading edges in thedirection of movement. Accordingly, to determine if star images enteringthe field of view have recognizable spectral signatures, an integratedimage need only be analyzed along swaths of the leading edges of theimaging area 28 for identifying trackable star images. Moreover,assuming the satellite or spacecraft has not lost its bearings, acandidate star image's “point of entry” in the field of view can beanticipated by using the star database 94 (FIG. 1) that associates starimage spectral characteristics with celestial locations. Note thatsatellite or spacecraft motion will typically not be aligned with apredetermined geometric axis for the star tracker of the presentinvention, and therefore, two adjacent leading edges are used foracquiring newly entered field of view star images. For example, thebottom and right edges of the imaging area 28 could constitute leadingedges for a particular movement of the satellite or spacecraftcontaining the present invention.

Accordingly, when searching for candidate star images, pixels withinswaths along the leading edges need only be analyzed for such starimages. Moreover, for each swath, its dimension perpendicular to theswath's associated leading edge needs to only span a sufficient numberof charge cell 36 rows and/or columns so that a candidate star image'smotion can be captured within a few frame integration times. Thus, it isan aspect of the present invention that most of the imaging area 28 canbe ignored in the acquisition mode. Furthermore, since high precisioncentroiding is not required in the acquisition mode, pixels obtained foruse in acquisition analysis may be binned into super-pixels of 2 by 2, 4by 4, or 8 by 8 pixels, thereby to increase the pixel read rate duringacquisition mode.

The flowchart of FIGS. 11A through 11C describes the steps performedduring acquisition mode. In particular, this flowchart relates to aspecific example, wherein it is assumed that: (a) the present inventionincludes a 512 by 512 split frame device with a 32 by 512 memory section42 at opposition ends of the split frame device (as in FIG. 1), (b) themotion of the satellite or spacecraft is such that star images willenter on either the right or bottom edges of the imaging area 28, (c)the binning operation is for providing super-pixels that are 8 by 8arrays of cells 36, (d) the swath along the bottom edge of the imagingarea 28 is 24 cell rows high, and (e) the swath along the right edge ofthe imaging area 28 has a width of 24 columns of cells 36.

Additionally note that in the flowchart of FIGS. 11, the extendedportions 54 a and 54 b (FIG. 1) are referenced, wherein such extendedportions are extra cells 36 attached to an end of an associated serialregister 46. Such extra cells of the extended portions are necessary forgaining physical room for needed electrical connections. Further notethat the cells 36 of such extended portions do not pair up in aone-to-one fashion with cells 36 from rows 32 of the memory sections 42.Moreover, note that a typical size of such an extended portion 54 iseight cells 36. Accordingly, in the description of FIG. 11 hereinbelow,such extended portions 54 a and 54 b may be assumed to be eight cells 36in length. However, it is within the scope of the present invention thatother sizes for the extended portions may be used, as one skilled in theart will understand.

Referring now to the steps of FIGS. 11, in step 1104, an image isintegrated on the imaging subareas 50 a, 50 b for a sufficient timeperiod so as to accumulate signal charges thereon large enough to havean adequate signal-to-noise ratio. In step 1108, the first or bottomeight pixel lines of the imaging subarea 50 b are shifted into the row32 b ₂ of the memory section 42 b while holding the first phaseelectrode of the memory section 42 b at a high potential so that thisrow becomes an accumulator for all eight of the pixel lines shifted outof the imaging subarea 50 b. Subsequently, in step 1112, the memorysection 42 b is shifted one row 32 toward the serial register 46 b. Instep 1124, a determination is made as to whether there are further pixellines in the imaging subarea 52 b. If so, then steps 1108 through 1124are iteratively performed until each consecutive set of eight pixellines from the imaging subarea 50 b are binned into a uniquelycorresponding pixel line of the memory section 42 b. Additionally, notethat the steps 1108 through 1124 have corresponding steps that apply toimaging subarea 50 a, and the memory section 42 a such that the pixellines of an integrated image in imaging subarea 50 a is similarlycompressed within the memory section 42 a.

Steps 1128 through 1148 are for reading and digitizing the pixels withinthe swath along the bottom edge of the imaging area 28. Accordingly, thesteps here have no counterparts to be performed using the imagingsubarea 50 a, the memory section 42 a and the serial register 46 a giventhe current illustrated location of the leading edges of the imagingarea 28. In step 1128, the serial register 46 b is cleared, andsubsequently in step 1132, the pixel lines provided within the memorysection 42 b are shifted toward the serial register 46 b so that thepixel line in row 32 b ₃ moves into the serial register 46 b.Subsequently, in step 1136, the pixels of the serial register 46 b areshifted so that the corresponding extended portion 54 b is filled withpixels from the serial register. In step 1140, the serial register 46 bis again shifted so that the extended portion is filled with the nextpixels in the serial register 46 b, while concurrently the on-chipcharge detection circuit 72 b: (a) reads the pixels being replaced inthe extended portion, and (b) outputs corresponding amplified signals tobe digitized by a signal receiving A/D converter 90 (after the signal isprocessed by the corresponding preamplifier 76, and analog processor80). Subsequently, in step 1144, a determination is made as to whetherthere are additional pixels in the serial register 46 b. If so, thensteps 1140 and 1144 are iteratively performed until all pixels withinthe serial register 46 b have been read and digitized. Subsequently,step 1148 is performed wherein a determination is made as to whetherthere is another pixel line in the memory section 46 b that representspart of the 24 pixel lines of the swath across the bottom of the imagingarea 28. Accordingly, if there are further such pixel lines to beprocessed, then steps 1132 through 1148 are iteratively performed untilall such pixel lines are processed.

Steps 1152 through 1172 of FIGS. 11 describe the processing performed onthe pixels of the swath along the right edge of the imaging area 28. Inparticular, these steps describe the processing performed using theimaging subarea 50 b, the memory section 42 b, the serial register 46 band associated circuitry such as on-chip charge detection circuit 72 b.However, note that since the right edge swath extends across the imagingsubarea 50 a as well, the steps 1152 through 1172 have correspondingsteps that are applicable to the imaging subarea 50 a, the memorysection 42 a, and the serial register 46 a plus related circuitry.Accordingly, in step 1152, the pixel lines of the memory section 42 bare shifted toward the serial register 46 b so that the pixel line inrow 32 b ₃ moves into the serial register 46 b. Subsequently, in step1156, the pixels in the serial register 46 b are shifted so that theextended portion 54 b is filled, and the reset transistor of the on-chipdetection circuit 72 b is set so that subsequent pixels received areread and output to be digitized. Following this, steps 1160 and 1164 areiteratively performed for reading and subsequently digitizing the pixelsof the right end of the serial register 46 b that were derived from theright edge swath. Accordingly, once the right edge swath pixels havebeen processed, step 1168 is performed wherein the remainder of thepixels of the serial register 46 b are flushed by rapid clocking, or byuse of a dump drain if such is provided within an embodiment of thepresent invention. Subsequently, in step 1172, a determination is madeas to whether there are additional pixel lines in the memory section 42b. If so, then steps 1152 through 1172 are iteratively performed forprocessing the pixels derived from the right edge swath while discardingor flushing pixels not derived from this swath. After all pixel lineswithin the memory section 42 b have been processed, step 1176 isperformed for waiting until another activation of step 1104 is complete,wherein another image has been integrated onto the imaging area 28.Subsequently, in step 1180, the flow of control is transferred to step1108 for processing pixel lines of another instance of the bottom andright edge swathes.

Note that once the steps 1104 through 1180 of FIGS. 11 have beenperformed iteratively a sufficient number of times, the centroid andprediction modules 88 are able to determine whether the spectralcharacteristics and approximate location of any new star image withinthe swaths of the leading edges match corresponding information forknown trackable stars in the star database 94.

Proceeding now to a more general description of the processing performedby the present invention, FIGS. 2 through FIG. 9 will now be described.

FIGS. 2A and 2B provide a description of the high level steps performedby the star tracking system controller 70 during operation of the startracker 20. Accordingly, on initial activation of the star tracker 20,the star tracking system controller 70 receives location and angularrotation rates from the inertial guidance system 78 (step 204), andsubsequently in step 208, the star tracking system controllerinterrogates the star database 94 for identifications of candidatesstars that are likely to become detectable on the CCD imaging area 28.Assuming the star tracking system controller 70 receives suchidentifications of the candidate stars, the star tracking-systemcontroller may select one or more such candidate stars (step 212) whosestar images are desired to be tracked. The selection process used by thestar tracking system controller 70 may include an analysis of thespectral characteristics (also denoted hereinbelow as a “signature”) ofthe candidate stars, wherein such characteristics are contained in thestar location data received from the star database 94. In particular,the star tracking system controller 70 may use an expected amplitude orintensity of each of various candidate stars for determining those starsthat the star tracker 20 will endeavor to track. Subsequently, in step216, the star tracking system controller 70 supplies the centroid andprediction modules 88 with signature data (e.g., including spectralamplitudes) indicative of each of the candidate stars to be acquired.Following this (step 220), the star tracking system controller 70provides instructions to the time base generator 66 indicating (a)through (c):

(a) An integration time for capturing an image in the imaging area 28,wherein this integration time is sufficiently long enough to obtain anadequate signal-to-noise ratio for the dimmest star to be tracked;

(b) The edges of the imaging area 28 where the candidate star image(s)is likely to appear. Note that such edges are no more than a single oneof the row edges 30 and 34, plus, no more than one of the column edges38 and 40 as described previously; and

(c) That the time base generator 66 is to enter a star acquisition modefor performing the steps of the flowchart of FIG. 3 describedhereinbelow.

Subsequently, in step 224, for the each of the candidate stars whosestar image is detected, the tracking system controller 70 requests fromthe centroid and prediction modules 88 a predicted centroid of the starimage for a subsequent integration on the imaging area 28. Given thisprediction information, the star tracking system controller 70, in step228, provides the time base generator 66 with (a) through (c) following:

(a) The predicted centroids of the stars to be tracked in the nextintegration;

(b) For each predicted centroid, the dimension(s) of a track-box 200 ofthe imaging area 28 in which the corresponding star image is highlylikely to be entirely included; and

(c) Instructions to enter a star tracking state or mode, wherein thesteps of FIG. 7 described hereinbelow are performed.

Note that in one embodiment, each such track-box is a square array of12×12 CCD cells 36. Further note that the size of such track-boxes (interms of CCD cells) is dependent upon the following characteristics ofthe star tracker 20:

(i) The angular subtense of each CCD cell 36,

(ii) The expected amount of spectral energy spread across the imagingarea 28 by each of the star images during an integration, and

(iii) Values indicative of the possible error in centroid predictionsfor subsequent image integrations.

Also note that regarding (ii) immediately above, the star imagesprovided on the imaging area 28 may be purposely defocused in order toprovide a larger image on the imaging area 28 of each star beingtracked. Such defocusing of star images has been found to yield, in somecases, more accurately computed star image centroids, and in particular,star images computed to an accuracy smaller than the CCD cells 36.

Subsequently, in step 232, the star tracking system controller 70 isperiodically alerted by the inertial guidance system 78 that there is anew satellite location and/or new angular rotation rates for thesatellite. Accordingly, the star tracking system controller 70 uses thisnew information to interrogate the star database 94 for new stars thatare likely to be detected on the imaging area 28. In step 236, the startracking system controller 70 subsequently determines whether there arenew star identifications to be retrieved from the star database 94 thatare likely to be detected. If there are no such new star identificationsretrieved, then the flow of control returns to step 232 to awaitadditional satellite location and/or angular rates of rotation data fromthe inertial guidance system 76. However, it should be noted that thestar tracking system controller 70 may be involved concurrently inperforming other processes related to star tracking when not activelyperforming step 232. In particular, the star tracking system controller70 may be actively involved in controlling the tracking of currentlydetected stars according to the processing performed in FIG. 7 describedhereinbelow.

If in decision step 236, the star tracking system controller 70determines. that there are new stars that may be detected, then step 240is performed wherein a determination is made as to. whether any of thenew stars are better candidates to be tracked than one or more of thestars being currently tracked. If not, then step 244 is performedwherein an additional determination is made as to whether any of thecurrently tracked stars are about to leave the imaging area 28. Thus, ifnone of the currently tracked stars are about to leave the imaging area28, then again, the flow of control returns to step 232. However, if oneor more currently tracked stars are about to leave the imaging area 28,then step 248 is performed wherein the star tracking system controller70 determines whether one or more of the new stars have images that areappropriate to be acquired (e.g., the images have a sufficiently highexpected spectral amplitude). Note that step 248 is also performed whenstep 240 results in an indication that one or more of the new stars thatmay be detected are determined to be better candidates for tracking thanone or more of the currently tracked stars.

Finally, note that once step 248 is performed, the star tracking systemcontroller 70 returns to step 216 in preparation for directing the timebase generator 66 to enter at a star image acquisition mode.

In FIG. 3, a flowchart is presented illustrating the high level stepsperformed during star acquisition once the star tracking systemcontroller 70 has instructed the time base generator 66 to enter a staracquisition mode or state. Accordingly, in step 304 of FIG. 3, the timebase generator 66 obtains from the star tracking system controller 70the adjacent edges of the imaging area 28 where one or more new stars tobe tracked are predicted to appear. Note that one of these edges will bea row edge 30 or 34, and the other edge will be one of the column edges38 or 40. In particular, the two edges provided are the edges where newfields of view are entering the imaging area 28. Subsequently, in step308 and step 312, the row edge and the column edge identified in step304 are assigned to the variables RE and CE, respectively, fornotational convenience. Additionally, in steps 316 and 320, the twoserial registers 46 a and 46 b are distinguished from one another fornotational convenience. In particular, the serial register adjacent rowedge RE is denoted by the identifier SR_(RE), and the serial register 46at the opposite end of the CCD 24 is denoted by the parameter SR_(OPP).In step 324, the time base generator 66 causes the imaging subareas 50 aand 50 b to be exposed to ambient spectral radiation for a sufficientlength of time to accumulate pixel charge in these imaging subareaslarge enough to have an adequate signal-to-noise ratio for identifyingthe new one or more stars. Note that the time for such spectralradiation exposure (also known as integration time) is provided by thestar tracking system controller 70, and this length of time may be, insome embodiments, a constant while in other embodiments it may bevariable that depending on the expected amplitudes of the spectralradiation emitted from the one or more stars whose images are expectedto be acquired during star acquisition.

During and/or following step 324, step 328 is performed wherein theserial registers 46 a and 46 b are cleared. Subsequently, following bothsteps 324 and 328, steps 332 through 352 are performed wherein theintegrated images in the imaging subareas 50 a and 50 b are firsttransferred to their respective memory sections 42 a and 42 b where eachof their corresponding images are compacted within their respectivereduced size memory sections. In particular, in steps 332 and 336, theprocess illustrated by the flowchart of FIG. 4 is performed forcompacting each of the images in the imaging subareas 50 a and 50 b intoits corresponding memory section 42 a and 42 b respectively. The processfor this compaction will be briefly discussed now. The flowchart of FIG.4 bins together pixel lines into the first cell row (i.e., 32 a ₁ or 32b ₁) of the corresponding memory section. More precisely, the time basegenerator 66 provides control signals (via lines 62) to each of theimaging subareas 50 a and 50 b so that each of these subareas shifttheir pixel lines toward their respective memory sections 42, and thepixels in the cell rows 32 a ₁, 32 b ₁ are transferred into the adjacentcell rows 32 a ₂ and 32 b ₂ respectively. Further, multiple contiguouspixel lines from the imaging subareas are binned within memory sectioncell rows 32 a ₂ and 32 b ₂ so that the images in the imaging subareas50 a and 50 b are compacted into their corresponding memory sections 42a and 42 b. That is, the pixel lines are binned in groups (e.g., eightpixel lines per group) as indicated in step 404 of FIG. 4. Subsequently,in step 408, once a group of consecutive pixel lines are binned, thetime base generator 66 instructs the corresponding memory section toshift the pixel lines of binned pixels one cell row 32 toward the serialregister 46 that receives output from the memory section. Followingthis, in step 412, a determination is made by the time base generator 66as to whether there are additional consequentive pixel lines to bin. Ifso, then steps 404 through 412 are repeated, if not then the program ofFIG. 4 terminates and returns to the corresponding step (either 332 or336 of FIG. 3) from which the program corresponding to FIG. 4 wasactivated.

Referring again to FIG. 3, once step 332 is performed, step 340 isencountered wherein the output of the binned row edge pixel line swathin the memory section MS_(RE) is output to the corresponding serialregister SR_(RE) by a process corresponding to the flowchart of FIG. 5described hereinbelow. Subsequently, step 344 is performed wherein thebinned column edge swath of pixels in the memory section MS_(RE) isoutput by the steps of FIG. 6, as will also be described hereinbelow.Additionally, note that since the column edge swath extends over bothimaging subareas 50 a and 50 b, step 344 is also performed on memorysection MS_(OPP) following step 336. Accordingly, steps 344 through 352are duplicately performed on each of the flow of control pathsbeginning, respectively, with the steps 332 and 336.

Following the performance of step 344, the output from the binned rowedge swath of pixels and the output from the binned column edge swath ofpixels have been supplied (via on-chip charge detection circuits 72 andcomponents 76, 80 and 90) to the centroid and prediction modules 88 foridentifying any of the candidate star images that may have entered thefield of view of the imaging area 28. Accordingly, in step 349 thecentroid and prediction modules 88 determine whether there is a clusterof pixel charges within one of the binned row swath or binned columnswath that matches the anticipated signal amplitude at the anticipatedlocation for any of the candidate stars for tracking. Accordingly, instep 352 the centroid and prediction modules 88 output to the trackingsystem controller 70 the identifiers for any stars whose signalamplitudes were identified. Subsequently, the process corresponding toFIG. 3 terminates and returns to step 220 of FIG. 2A.

As mentioned hereinabove, FIG. 5 provides a high level description ofthe steps performed by each memory section 42 a and 42 b in coordinationwith its corresponding serial register 46 a and 46 b, respectively, foroutputting a binned row edge swath of pixels. Accordingly, in step 504of FIG. 5, the serial register, (denoted herein as SR) for the memorysection having the row edge swath of pixels, is cleared. Subsequently,in step 508, the pixel line within the adjacent memory section cell row(one of 32 a ₃ and 32 b ₃) is transferred into the serial register SR.Following this, in step 512, the serial register SR is instructed by thetime base generator 66 to parallelly shift its cell charges (e.g. pixelcharges) toward the extended positions 54 of the serial register SR, andclear the detection node of the on-chip charge detection circuit 72receiving output from SR. Subsequently in step 516, signalscorresponding to the pixel charges within the extended register portion54 are transferred (via components 72, 76, and 80) to the A/D converter90 so that these signals may be digitized according to signal amplitudeas indicated in step 520. Subsequently, in step 524, the A/D converter90 outputs the corresponding digitized data to the centroid andprediction modules 88 and in step 528, the on-chip charge detectioncircuit 72 waits for a next group of pixels to be read from the extendedportion 54 of the serial register SR, wherein once such further signalscorresponding to the binned pixel values are available, step 520 isagain performed.

In addition to, and at least somewhat independent of, the processingperformed in steps 520 through 528, step 532 is performed by the timebase generator 66 for determining whether there is additional pixel datain the serial register SR. Accordingly, if there is, then step 512 (andsubsequent steps) are performed as discussed above. Alternatively, ifall such pixel data in serial register SR has been read (via the on-chipcharge detection circuit 72), then step 536 is performed for determiningwhether there is another pixel line in the memory section having rowedge swath binned data therein. If so, then steps 504 (and subsequentsteps) are again performed. Alternatively, the process corresponding tothe flowchart of FIG. 5 terminates and the flow of control returns tostep 340 of FIG. 3.

FIGS. 6A and 6B provide a flowchart corresponding to the high levelsteps performed by the present invention for outputting the portion ofthe binned column edge swath of pixels contained in one of the memorysections 42 a and 42 b. In particular, the flowchart of FIGS. 6A and 6Bis performed for each of the memory sections 42 a and 42 b when thecolumn swath pixel data therein is output to the respectivecorresponding serial register 46 a and 46 b. Accordingly, in step 604, adetermination is made as to whether the column edge (CE) for this columnswath of pixels is near the output end of the memory section serialregister (SR), or, whether CE is adjacent the opposite end of thisserial register. Assuming the time base generator 66 determines that thecolumn edge is near the output end of the serial register SR, then step608 is performed wherein the time base generator 66 causes the memorysection to transfer the pixel charges in the adjacent row cell (i.e.,either 36 a ₃ or 36 b ₃) into the serial register SR. Subsequently, instep 612, the time base generator 66 instructs the serial register SR toshift its pixel charges the number of extended positions in the extendedportion 54 of SR and clear the detection node of the on-chip chargedetection circuit 72 receiving output from SR. In step 616, signalscorresponding to the pixel charges within the extended portion 54 of SRare output to the on-chip charge detection circuit 72, which undercontrol of the star tracking system controller 70, outputs suchcorresponding signals to either the drain 86, or one of thepreamplifiers 76. In particular, since the swath is on the edge whosepixels are first read into the extended portion 541 in step 620 thefirst collection of pixels read into this extended portion have theircorresponding signals eventually digitized by an A/D converter 90 andthe resulting digitized output is provided to the centroid andprediction modules 88 in step 624. Subsequently, in step 628, adetermination is made by the time base generator 66 as to whether thepixels of the column edge swath have been completely output. If not,then step 620 is again encountered for subsequently digitizing andoutputting the signals (corresponding to the swath edge pixels) to thecentroid and prediction modules 88. Alternatively, if the time basegenerator 66 determines that all pixels of the swath have been read,then in step 632 the remainder of the pixel charges in the serialregister SR are purged or flushed from SR. Subsequently, in step 636,the time base generator 66 determines if there is another pixel line inthe memory section having binned swath charges. If so, then the flow ofcontrol is transferred back to step 608 for processing the next suchpixel line. Alternatively, if the result of step 636 is negative thenall pixel lines in the memory section have been processed. Accordingly,the process corresponding to the flowchart of FIG. 6 terminates and areturn is made to the process corresponding to the flowchart of FIG. 3at step 344.

Referring again to Step 604, if the swath for the column edge CEprovides pixel charges at the opposite end of the serial register SRfrom the end having the extension portion 54, then step 644 is nextencountered wherein a pixel line from the cell row 32 that outputs tothe serial register SR is transferred into SR. Subsequently, in step648, the time base generator 66 shifts the serial register SR the numberof extended positions in portion 54 and clears the detection node of theon-chip charge detection circuit 72 receiving output from SR. In step652, the time base generator 66 causes the pixel charges of the serialregister SR at the beginning of the serial register to be purged orflushed. Subsequently, all remaining pixel charges in the serialregister SR are those from the swath. Accordingly, in step 656, theextended positions of serial register portion 54 are read and theircorresponding signals are transferred (eventually) to A/D converter 90so that their signal amplitudes can be digitized as indicated in step660. Subsequently, in step 664, the A/D converter 90 outputscorresponding digitized data to the centroid and prediction modules 88for storing until an image of the entire swath is obtained which thenallows a determination to be made as to whether any of the expected starimages have been detected. Additionally, in step 668, the A/D converter90 waits for further pixel data to be obtained from the on-chip chargedetection circuit 72, and when such data is obtained step 660 is againactivated for digitizing this data. In parallel and coordinated withperforming the steps 660 through 668, the time base generator 66determines in step 672 whether there is more pixel data to be read fromthe serial register SR. If so, then in step 676 the time base generator66 causes the serial register SR to shift its pixel charges toward theextended portion 54 the number of cells within this extended portion.Then step 656 can again be performed for thereby outputting thecorresponding signals for the pixel charges within extended portion 54.

Referring again to steps 672, if it is determined that there are nofurther pixels in the serial register SR, then step 680 is performed,wherein the time base generator 66 determines if there is another pixelline in the memory section having binned swath charges. If so, then step644 is again encountered to process this new pixel line. Alternatively,the processing of pixel lines in the image section is complete, and theprocess corresponding to FIGS. 6A and 6B terminates with a return to thestep 344 of FIG. 3 occurs.

FIG. 7 is a flowchart of the high level steps for tracking stars onceacquisition of their star images has been obtained. Accordingly, in step704, star tracking commences with the integration of spectral radiationon the imaging area 28 for a sufficient period of time to accumulatecell charges large enough to have an adequate signal-to-noise ratio onthe dimmest star being currently tracked. After the integration timeperiod has expired, in step 708, each imaging subarea 50 a and 50 btransfers the pixel lines of its accumulated image to the respectivecorresponding memory section 42 a and 42 b, wherein the image iscompacted by binning together its pixel lines that do not intersect anytrack-box 200 (FIG. 1). Note that detailed steps for performing thisprocess are provided in FIG. 8 discussed hereinbelow. Once the images ofthe imaging subareas 50 a and 50 b have been transferred to theirrespective corresponding memory sections 42 a and 42 b, the time basegenerator 66 subsequently configures the imaging area 28 to againperform step 704, for integrating spectral radiation received on theimaging area 28. Note that in one embodiment of the present invention,approximately 100 integrations per second are performed. Additionally,in step 712 following step 708, the time base generator 66 directs eachmemory section 42 a and 42 b to transfer each pixel charge of atrack-box to the corresponding serial register 46 to which the memorysection outputs, and consequently such pixel charges are digitized andprovided to the centroid and prediction modules 88. Note that thedetails of step 712 are provided in FIG. 9 discussed hereinbelow. Instep 716, the time base generator 66 delays any further memory sectionprocessing until there are additional pixels available for transfer fromthe imaging area 28. In parallel with step 716, step 720 may beperformed wherein the centroid and prediction modules determine whetherany track-box digitized pixel data has been obtained. Accordingly, ifsuch data has not been obtained, then step 724 is performed wherein thecentroid and prediction modules 88 wait for track-box data to besupplied via one of the A/D converters 90. Alternatively, if track-boxpixel data is supplied to the centroid and prediction modules 88, thenin step 728, the centroid and prediction modules 88 determine the chargeamplitude centroid for a first of the track-boxes whose data has beeninput thereto. Subsequently, in step 732, the centroid and predictionmodules 88 predict a new centroid location on the imaging area 28 forthe star image whose centroid was calculated in step 728. Note that thispredicted new centroid location is determined using the angular rate ofrotation of the satellite as supplied by the inertial guidance system78. In step 736, the centroid and prediction modules 88 determinewhether the newly predicted centroid is located on the imaging area 28.If it is determined that the predicted centroid is not on the imagingarea 28, then step 740, the star tracking system controller 70 isalerted that the corresponding star can no longer be tracked.Alternatively, if the results at step 736 indicate that the predictednew centroid is in the imaging area 28, then step 744 is performed foroutputting the predicted centroid to the star tracking controller 70.Subsequently, in step 748, a determination is made by the star trackingcontroller 70 as to whether continued tracking of the star identified bythe newly predicted centroid should occur. If so, then in step 752, thestar tracking controller 70 instructs the time base generator 66 thatthe pixel values within a track-box 200 centered about the newlypredicted centroid are to be output from the imaging area 28 withoutbeing compacted or binned within one of the memory sections 42 a and 42b. Subsequently, regardless of the path taken from step 736 fordetermining how to use the newly used predicted centroid, in step 756, adetermination is made by the centroid and prediction modules 88 as towhether there is additional track-box pixel data from which a newcentroid can be determined. Accordingly, if such additional data isavailable, then step 728 is again performed for calculating a currentand a predicted centroid for the new track-box data. Alternatively, ifno such further track-box data is available, then step 724 is performedwherein the centroid and prediction modules 88 wait for such data.

In FIG. 8, a flowchart of the high level steps performed by one of theimaging subareas (50 a or 50 b), and the corresponding memory section(42 a or 42 b) when these components are controlled by the time basegenerator 66 for transferring an image from the imaging subarea(denoted, “IA”) to its corresponding memory section. Accordingly, instep 804, the memory section (42 a or 42 b) corresponding to the imagingsubarea is denoted by the identifier MS. Additionally, in step 808, theserial register 46 to which the memory section MS outputs its pixellines is identified by the identifier SR. Assuming that the imagingsubarea denoted by IA has an image integrated thereon, in step 812, thetime base generator 66 determines whether there are any track-boxes 200provided within the imaging subarea IA, and if so, then the time basegenerator determines which of the one or more track-boxes is closest tothe memory section MS. In particular, the time base generator 66determines the number of cell rows 32 between the (any) track-box 200closest to MS, and the memory section MS, this value being assigned fornotational convenience to the variable “ROW_TRANSFERS.” Subsequently, instep 816, the time base generator 66 provides control signals to theimaging subarea IA (via lines 62) for parallelly shifting the pixellines of IA ROW-TRANSFERS number of cell rows 32 (assuming at leasttrack-box exists in IA). Moreover, note that the time base generator 66maintains an electrical configuration of the memory section MS duringthis shifting of the AI pixel lines so that the pixel lines shifted intothe memory section are binned together into the first cell row 32 thatreceives the pixel lines (i.e., one of the cell rows 32 a ₂ and 32 b ₂).In step 820, the time base generator 66 causes the memory section MS toshift the pixel lines therein a predetermined number of cell rows 32toward the serial register SR to thereby obtain one or more guard bandpixel lines that are used for insulating subsequent pixel lines to beread into the memory section MS from the binned pixel line resultingfrom step 816. As a result of seeps 816 and 820, the configuration ofthe imaging subarea IA is such that there is a track-box 200intersecting the imaging subarea cell row adjacent the memory section MS(i.e., either cell row 32 a ₂ or 32 b ₂), and additionally, the binnedpixel line has been shifted further into MS so that guard band linesprovide a place to accumulate dark current and background signal chargebetween the binned pixel line and subsequent pixel lines transferredinto the memory section MS. Accordingly, in step 824, the time basegenerator 66 synchronously shifts the pixel lines of both the imagingsubarea IA and the memory section MS toward the corresponding serialregister SR, wherein the pixel lines are shifted the number of cell rows32 necessary to completely shift into MS the track-box(es) 200 thatpreviously intersected the cell row 32 immediately adjacent the memorysection MS. Accordingly, the trackbox(es) 200 are effectively duplicatedwithin the memory section MS. It is worth noting in this context thatthe track-box(es) 200 may be aligned as in FIG. 1. That is, the edges ofthe track-boxes 200 align with CCD cell 36 rows and columns of theimaging area 28. However, it is within the scope of the presentinvention that track-boxes 200 may have geometries other than squares;in particular, circles, triangles and other polygonal regions may alsobe utilized by the present invention. Additionally, it is within thescope of the present invention to also use amorphously defined regionsas track-boxes. Further, it is worthwhile to note that since the starsto be tracked can be selectively chosen, and since during tracking,track-boxes 200 retain their relative distances to one another, starscan be chosen for tracking wherein their corresponding track-boxes 200are sufficiently spaced apart on the imaging area 28 so that any twosuch track-boxes 200 either have identical cell rows 32, or moretypically, they have no cell row 32 in common.

In step 828, the time base generator 66 determines if there is anadditional track-box 200 to be shifted into the memory section MS.Accordingly, if there is, then step 832 is performed wherein the pixellines of the memory section MS are shifted a predetermined number ofcell rows 32 toward the serial register SR to obtain additionalinsulating guard band lines substantially as in step 820. Subsequently,in step 836, the time base generator 66 determines the number of cellrows 32 between the memory section MS and the closest track-box 200thereto in the imaging subarea IA. Subsequently, step 816 and stepsfollowing are again performed for binning those cell rows 32 notintersecting a track-box 200, and then (step 824) duplicating in MSthose pixel lines that intersect a next track-box 200. It is noteworthythat the number of iterations through the loop of steps 816 through 832is bounded by the number of cell rows 32 in the memory section MS.Moreover, the maximal number of iterations of this loop is equal to themaximal number of track-boxes 200 (having non-intersecting cell rows) inIA. The number of cell rows 32 in the MS is also a function of thenumber of guard band lines. A guard band line needs to lead the firsttrack box and trail the last track box. These guard band lines provide aplace to accumulate dark current and background signal charge. Guardband lines should also be apportioned based on the number of linesbetween track boxes and how far a track box is located from the top andbottom edges as appropriate.

Referring again to decision step 828, if there are no additionaltrack-boxes 200 to be shifted into the memory section MS, then decisionstep 842 is performed wherein it is determined whether there are furtherpixel lines in the imaging subarea IA not already binned together.Accordingly, if such pixel lines remain in the imaging subarea IA, thenstep 846 is performed, wherein additional guard band lines are providedwithin the memory section MS, and subsequently in step 850, theremaining pixel lines in the imaging subarea IA are binned together inthe adjacent cell row of the memory section MS (i.e., either cell row 32a ₂ or 32 b ₂). Subsequently, the process corresponding to FIG. 8terminates and the flow of control returns to step 708 of FIG. 7.Alternatively, if in step 842, there are no further pixel lines in IA tobe processed, then the process corresponding to FIG. 8 also terminatesand the flow of control returns to step 708 of FIG. 7.

FIG. 9 illustrates a high level flowchart of the steps performed whenpixel charges are transferred from one of the serial registers 46 a and46 b through the respective one of the on-chip charge detection circuit72 a and 72 b for either purging, or being subsequently processed by thecentroid and prediction modules 88. In particular, in step 904, for agiven one of the memory sections 42 a and 42 b (denoted MS), itscorresponding serial register is denoted by the identifier SR.Subsequently, in step 908, any pixel lines in the memory section MS thatdo not include pixel charges from a track-box 200 are purged or dumped.Accordingly, upon completion of step 912, the serial register SRincludes a pixel line that has track-box 200-data. Thus, in step 916,the time base generator 66 causes the serial register SR to rapidlyshift into the extended portion 54 until the first track-box 200 datawithin the pixel line is available in the extended portion 54 forreading via a corresponding one of the on-chip charge detection circuits72. Note that in one embodiment, when rapidly shifting the pixels inserial register SR, the corresponding on-chip charge detection circuitis configured to output such pixel charges to the drain 86.Alternatively, upon outputting of track-box 200 pixel charges from theextended portion 54, the corresponding on-chip charge detection circuitis configured to supply pixels) to the A/D converter 90 for digitizingamplitudes of the signals(step 920). The A/D converter 90 then outputsits digital data to the centroid and prediction modules 88 in step 924,and simultaneously, the time base generator 66 determines in decisionstep 928 whether there is an additional contiguous series of pixelcharges within the serial register SR for another track-box 200. If so,then step 916 is again performed. Alternatively in the more typical case(where track-boxes do not have pixel lines in common), step 932 isperformed wherein the remaining pixel charges in the serial register SRare flushed or dumped to the drain 86 and subsequently in decision step936, the time base generator 66 determines whether there are additionalpixel lines in the memory section MS that may include track-box 200data. If so, then step 908 and subsequent steps are again iterativelyprocessed to digitize the additional track-box 200 data and provide thedigitized version thereof to the centroid and prediction modules 88.Alternatively, if all such pixel lines remaining in the memory sectionMS contain no track-box 200 data, then from the step 936, the processcorresponding to FIG. 9 terminates and the flow of control returns tostep 712 of FIG. 7.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedhereinabove are further intended to explain best modes known forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

What is claimed is:
 1. A method for collecting location information foran object, comprising; obtaining location information that is used indetermining a location of the object, said location information beingobtained using a first number of lines of an image section of a chargecoupled device; obtaining discardable information that is not used indetermining the location of the object, said discardable informationbeing obtained using a second number of lines of said image section ofsaid charge coupled device; providing said location information and saiddiscardable information to a memory section having a third number oflines, said third number of lines including a first set and a secondset, said location information being provided to said first set and saiddiscardable information being provided to said second set and in whichsaid second set has a number of lines that is less than said secondnumber of lines.
 2. A method, as claimed in claim 1, wherein: said thirdnumber of lens is less than 50% of the total of said first number oflines and said second number of lines.
 3. A method, as claimed in claim1, wherein: said third number of lines is less than 75% of the total ofsaid first number of lines and said second number of lines.
 4. A method,as claimed in claim 1, wherein: said step of obtaining said locationinformation includes defining at least a first track box comprising asubarray of said image section, said first track box including at leastportions of some of said first number of lines.
 5. A method, as claimedin claim 1, wherein: said step of providing said discardable informationincludes binning said discardable information into said second set andin which said second set includes one or more lines of said memorysection.
 6. A method, as claimed in claim 1, wherein: said step ofproviding said location information includes establishing at least afirst guard line in said memory section before said step of providingsaid discardable information.
 7. A method, as claimed in claim 1,further including: receiving said location information in a serialregister from said memory section, moving said location information intoan extended portion in communication with said serial register anddetecting pixel charges from said extended portion.
 8. A method forcollecting information related to a location of an object, comprising:obtaining desirable information that is used in determining a locationof an object and discardable information that is not used in determiningsaid location of the object using a first number of lines of a chargecoupled device; and providing said desirable information and saiddiscardable information to a second number of lines of a memory sectionof said charge coupled device, with said second number of lines beingless than said first number of lines.
 9. A method, as claimed in claim8, wherein: said second number of lines is less than 50% of said firstnumber of lines.
 10. A method, as claimed in claim 8, wherein: saiddesirable information includes one of location information related to alocation of the object and acquisition information related to whether ornot the object is present.
 11. A method, as claimed in claim 8, wherein:said first number of lines includes a third number of lines and a fourthnumber of lines, with said discardable information being obtained usingsaid fourth number of lines, said second number of lines includes afifth number of lines and a sixth number of lines, with said discardableinformation being provided to said sixth number of lines and said sixthnumber of lines being less than said fourth number of lines.
 12. Amethod, as claimed in claim 8, wherein: said desirable informationincludes acquisition information, said acquisition information isobtained from adjacent at least a first edge of said image section andsaid discardable information is obtained inwardly of said first edge.13. A method, as claim in claim 8, wherein: said desirable informationincludes acquisition information related to detecting a presence of theobject and each of said first and second lines includes desirableinformation and discardable information.
 14. A method, as claimed inclaim 8, wherein: said providing step includes producing a guard line insaid memory section intermediate said desirable information and saiddiscardable information.
 15. A charge coupled device for providinginformation related to location of an object, comprising: an imagesection including a first number of lines that has desirable informationthat is used in determining a location of an object, with said desirableinformation including acquisition information related to whether or notthe object is present and discardable information that is not used indetermining the location of the object; a memory section incommunication with said image section and including a second number oflines that is less than said first number of lines, said second numberof lines receives said desirable information and said discardableinformation.
 16. A charge coupled device, as claimed in claim 15,wherein: said second number of lines is less than 50% of said firstnumber of lines.
 17. A charge coupled device for providing informationrelated to location of an object, comprising: an image section includinga first number of lines that has desirable information that is used indetermining a location of an object and discardable information that isnot used in determining the location of the object; and a memory sectionincluding a second number of lines that is less than said first numberof lines, said second number of lines for receiving said desirableinformation and said discardable information; wherein said desirableinformation in said image section is present in a third number of linesand said discardable information is present in a fourth number of linesof said first number of lines and said desirable information is providedto said memory section in a fifth number of lines and said discardableinformation is provided to said memory section in a sixth number of oneor more lines of said second number of lines, said sixth number of oneor more lines being at least less than 50% of said fourth number oflines.
 18. A charge coupled device, as claimed in claim 17, wherein:said memory section includes at least a first guard line locatedintermediate said fifth number of lines and said sixth number of lines.19. A charge coupled device for providing information related tolocation of an object, comprising: an image section including a firstnumber of lines that has desirable information that is used indetermining a location of an object and discardable information that isnot used in determining the location of the object, said desirableinformation being present adjacent to at least a first edge of saidimage section and said discardable information being located inwardly ofsaid first edge; and a memory section including a second number of linesthat is less than said first number of lines, said second number oflines for receiving said desirable information and said discardableinformation.